When metal strip is produced, for instance by hot rolling in a mill, the strip's edges are not perfectly straight and its width varies somewhat. Thus it is necessary to trim off the edges to make them straight and give the strip a uniform width. The standard setup for doing this uses two trimmer heads each comprising a pair of cutter wheels between which the strip is passed, thereby cutting a trim strip from each edge. The two heads are spaced apart by the desired strip width and equipment is provided to center the incoming strip between them, ensuring under normal circumstances that both of the strip edges will be cut off fairly uniformly to produce the desired straight and uniform strip. Each trimming head is further associated with a winding or catching device which traps the cut-off trim strip and conducts it away. This system works perfectly so long as the incoming strip is generally straight and its width does not vary excessively.
It is not uncommon during normal production for a strip to have substantial camber, that is to have edges that are parallel but not straight, or for a strip to have a region of substantially substandard width. Furthermore it is standard in continuously running operations, for example pickling, coating, or annealing, for the leading end of a new strip to be attached by welding to the trailing end of a strip being processed so that the new strip need not be painstakingly threaded through the equipment. In this latter case there is frequently a sudden change in strip width or a sudden lateral dislocation of the center of the strip. When this happens it is possible for one of the trimming heads to run off the strip which in itself is not harmful, but the trim strip produced by the head runs out and when the head comes back onto the workpiece a new trim strip is started that can get fouled in the downstream equipment or that at least will not be collected in the usual manner. Obviously this can lead to substantial down time while the equipment is adjusted and the mess is cleared.